1. Field of the Invention
The present invention relates to a differential current protection circuit breaker that uses a magnetic amplifier circuit.
2. Description of Related Art
A known differential current protection circuit breaker has a secondary winding that connects a complex, predominantly capacitive measuring impedance in series with a square wave generator. A tap is connected to an evaluation circuit which actuates a switch contact device via a tripping device that acts as a latch. A differential current protective circuit breaker of this type is disclosed in West German Patent No. 3,424,959.
A differential current protection circuit breaker magnetically saturates a secondary winding of a magnetic amplifier (transductor) circuit of a summing current transformer with a.c. current using a high-frequency a-c voltage. A magnetic field is superimposed in the summing current transformer if a differential current flows through the primary winding. This magnetic field has the same direction as the applied field and exceeds the saturation limit of the transformer so that the inductive impedance of the secondary winding decreases. A measuring impedance that is in series with the secondary winding measures a higher potential relative to the reference potential from the square wave generator. The voltage amplitude at the measuring distance also increases. An evaluation circuit can compare the voltage amplitude with a predetermined threshold voltage. A tripping device can be activated and the monitored lines interrupted if the threshold voltage is exceeded. In addition to a-c currents, this type of differential current protection circuit breaker responds to all fault currents, such as to fault currents with a d-c component and to a smooth d-c fault current. This type of a differential current protection circuit breaker is effectively sensitive to all kinds of currents.
The summing current transformer requires few primary windings because of the complex, predominantly capacitive measuring impedance of the frequency of the squarewave generator. The small number of windings means that the heat of the summing current transformer is easily removed even in a small amount of space. A small protection circuit breaker can therefore be constructed as shown in West German Patent No. 3,424,959.
This type of sensitive differential current protection circuit breaker has a tendency to erroneously trip in response to current or voltage surges. west German Patent 3,429,381 provides for voltage limitation and subsequent integration to increase the resistance to current surges. A Zener diode contacts the reference potential and the secondary winding and a damping resistor is inserted ahead of the measuring impedance. This device nevertheless can trip in response to a current surge.
It is known that a high level of magnetic saturation of a ferromagnetic material causes a remanence increase over stationary operation for a short time lasting between 10 to 30 ms. The protection circuit breaker trips because the inductive impedance of the secondary winding is lower during the increased remanence induction and the voltage signal at the measuring impedance is therefore increased. A known differential current protection circuit breaker suppresses surge currents caused by remote lighting strikes and by switching processes in supply networks.